Wireless communication has evolved rapidly in the past decades as a demand for higher data rates and better quality of service has been continually required by a growing number of end users. Next-generation (so-called “5G”) cellular systems are expected to operate at higher frequencies (e.g., millimeter-wavelength or “mmW”) such as 5-300 GHz. Such systems are also expected to utilize a variety of multi-antenna technology (e.g., antenna arrays) at the transmitter, the receiver, or both. In the field of wireless communications, multi-antenna technology can comprise a plurality of antennas in combination with advanced signal processing techniques (e.g., beamforming). Multi-antenna technology can be used to improve various aspects of a communication system, including system capacity (e.g., more users per unit bandwidth per unit area), coverage (e.g., larger area for given bandwidth and number of users), and increased per-user data rate (e.g., in a given bandwidth and area). Directional antennas can also ensure better wireless links as a mobile or fixed devices experiences a time-varying channel.
The availability of multiple antennas at the transmitter and/or the receiver can be utilized in different ways to achieve different goals. For example, multiple antennas at the transmitter and/or the receiver can be used to provide additional diversity against radio channel fading. To achieve such diversity, the channels experienced by the different antennas should have low mutual correlation, e.g., a sufficiently large antenna spacing (“spatial diversity”) and/or different polarization directions (“polarization diversity”). Historically, the most common multi-antenna configuration has been the use of multiple antennas at the receiver side, which is commonly referred to as “receive diversity.” Alternately and/or in addition, multiple antennas can be used in the transmitter to achieve transmit diversity. A multi-antenna transmitter can achieve diversity even without any knowledge of the channels between the transmitter and the receiver, so long as there is low mutual correlation between the channels of the different transmit antennas.
In various wireless communication systems, such as cellular systems, there can be fewer constraints on the complexity of the base station compared to the terminal or the mobile unit. In such exemplary cases, a transmit diversity may be feasible in the downlink (i.e., base station to terminal) only and, in fact, may provide a way to simplify the receiver in the terminal. In the uplink (i.e., terminal to base station) direction, due to a complexity of multiple transmit antennas, it may be preferable to achieve diversity by using a single transmit antenna in the terminal multiple receive antennas at the base station.
In other exemplary configurations, multiple antennas at the transmitter and/or the receiver can be used to shape or “form” the overall antenna beam (e.g., transmit and/or receive beam, respectively) in a certain way, with the general goal being to improve the received signal-to-interference-plus-noise ratio (SINR) and, ultimately, system capacity and/or coverage. This can be done, for example, by maximizing the overall antenna gain in the direction of the target receiver or transmitter or by suppressing specific dominant interfering signals. In general, beamforming can increase the signal strength at the receiver in proportion to the number of transmit antennas. Beamforming can be based either on high or low fading correlation between the antennas. High mutual antenna correlation can typically result from a small distance between antennas in an array. In such exemplary conditions, beamforming can boost the received signal strength but does not provide any diversity against radio-channel fading. On the other hand, low mutual antenna correlation typically can result from either a sufficiently large inter-antenna spacing or different polarization directions in the array. If some knowledge of the downlink channels of the different transmit antennas (e.g., the relative channel phases) is available at the transmitter, multiple transmit antennas with low mutual correlation can both provide diversity, and also shape the antenna beam in the direction of the target receiver and/or transmitter.
In other exemplary configurations, multiple antennas at both the transmitter and the receiver can further improve the SINR and/or achieve an additional diversity against fading compared to only multiple receive antennas or multiple transmit antennas. This can be useful in relatively poor channels that are limited, for example, by interference and/or noise (e.g., high user load or near cell edge). In relatively good channel conditions, however, the capacity of the channel becomes saturated such that further improving the SINR provides limited increases in capacity. In such cases, using multiple antennas at both the transmitter and the receiver can be used to create multiple parallel communication “channels” over the radio interface. This can facilitate a highly efficient utilization of both the available transmit power and the available bandwidth resulting in, e.g., very high data rates within a limited bandwidth without a disproportionate degradation in coverage. For example, under certain exemplary conditions, the channel capacity can increase linearly with the number of antennas and avoid saturation in the data capacity and/or rates. These techniques are commonly referred to as “spatial multiplexing” or multiple-input, multiple-output (MIMO) antenna processing.
In order to achieve these performance gains, MIMO generally provides that both the transmitter and receiver have knowledge of the channel from each transmit antenna to each receive antenna. In some exemplary embodiments, this can be done by the receiver measuring the amplitude and phase of a known transmitted data symbol (e.g., a pilot symbol) and sending these measurements to the transmitter as “channel state information” (CSI). CSI may include, for example, amplitude and/or phase of the channel at one or more frequencies, amplitude and/or phase of time-domain multipath components of the signal via the channel, direction of arrival of multipath components of the signal via the channel, and other metrics known by persons of ordinary skill.
As used herein, “multipath component” can describe any resolvable signal component arriving at a receiver or incident on an antenna array at the receiver. The multipath component can be processed by the receiver at the radio frequency (RF), after conversion to an intermediate frequency (IF), or after conversion to baseband (i.e., zero or near-zero frequency). A plurality of the multipath components can comprise a main component of a transmitted signal received via a primary, direct, or near-direct path from the transmitter to the receiver, as well as one or more secondary components of the transmitted signal received via one or more secondary paths involving reflection, diffraction, scattering, delay, attenuation, and/or phase shift of the transmitted signal. Persons of ordinary skill can recognize that the number and characteristics of the multipath components available to be processed by a receiver can depend on various factors including, e.g., transmit and receive antennas, channel and/or propagation characteristics, transmission frequencies, signal bandwidths, etc.
In the case of a transmit array comprising NT antennas and a receive array comprising NR antennas, the receiver can be used to send CSI for NT·NR channels to the transmitter. Moreover, in mobile communication environments, these NT·NR channels are likely not stationary but vary according to the relative motion between the transmitter and the receiver (e.g., base station and terminal). The rate of change of the channel—and thus the preferable CSI update rate—can be proportional to the relative velocity between the transmitter and the receiver, and the carrier frequency of the signal being transmitted. Further mobile communication systems—including so-called “fifth-generation” or “5G” systems—can utilize mmW frequencies in the 5-300 GHz spectrum, which are substantially higher than the 1-5 GHz spectrum used by today's systems. In addition, increasing the numbers antennas (i.e., NT and/or NR) is expected to be an important technique for achieving performance goals for 5G systems including high data rates. In fact, as such mmW systems evolve, both the base stations and terminals could potentially utilize a multitude of antenna elements each, with the actual number of elements limited only by the physical area or volume available in each particular application.
Long Term Evolution (LTE) is an umbrella term for so-called fourth-generation (4G) radio access technologies developed within the Third-Generation Partnership Project (3GPP) and initially standardized in Releases 8 and 9, also known as Evolved UTRAN (E-UTRAN). LTE is targeted at various licensed frequency bands, including the 700-MHz band in the United States. LTE is accompanied by improvements to non-radio aspects commonly referred to as System Architecture Evolution (SAE), which includes Evolved Packet Core (EPC) network. LTE continues to evolve through subsequent releases. One of the features of Release 11 is an enhanced Physical Downlink Control Channel (ePDCCH), which has the goals of increasing capacity and improving spatial reuse of control channel resources, improving inter-cell interference coordination (ICIC), and supporting antenna beamforming and/or transmit diversity for control channel.
While LTE was primarily designed for user-to-user communications, 5G cellular networks are envisioned to support both high single-user data rates (e.g., 1 Gb/s) and large-scale, machine-to-machine communication involving short, bursty transmissions from many different devices that share the frequency bandwidth. Cellular systems depend on careful scheduling between multiple users to efficiently use the radio link and achieve high levels of spatial reuse. This scheduling demands significant control messaging overhead, which grows with the number of devices. This control messaging overhead includes the CSI reporting discussed above, which becomes doubly expensive as both the number of devices and the number of antenna elements per device increases.
In addition, 5G networks are envisioned to support various applications related to healthcare, logistics, automotive applications, virtual or augmented reality, and mission-critical control. These applications have a common requirement of round-trip (base to UE and back) radio link latencies of approximately 1 ms. This ultra-low latency target is at least an order of magnitude faster than the 10-ms minimum latency currently offered by LTE.
Design of the Medium Access Control (MAC) layer for 5G mmW cellular systems presents unique challenges due to the need for highly directional antennas at both the base station and terminal devices to overcome mmW propagation losses, low latency requirements, and a large number of heterogeneous devices with a wide range of data rates.
Accordingly, it can be beneficial to address at least some of these issues and/or problems with an improved MAC layer design.